Lattice panel structures

ABSTRACT

A modular system for the construction of a lattice panel for a bridge, comprises first and second chord members ( 20, 28 ) and a plurality of web members ( 1 ). Each web member comprises three legs forming a triangle. One leg ( 2 ) extends perpendicularly between the chord members. The second and third legs ( 3, 4 ) are joined to each other and to the ends of the first leg. The ends of the first leg ( 1 ) are provided with means for connecting the web member to the chords ( 20, 28 ). Interlocking means ( 13, 19 ) are provided at the joint between the second and third legs, and mid way along the first leg, so that adjacent web members can be connected together.

[0001] This invention relates to lattice panel structures, such asbridges of the “Bailey” type. The invention is particularly concernedwith a modular lattice panel system.

[0002] In a typical “Bailey” bridge construction there is provided amain girder at each side of the bridge, transoms extending between thegirders and a deck supported on the transoms. The girders are formedfrom prefabricated lattice panels of steel which are Joined togetherlongitudinally Two or more sets of the lattice panels may be securedtogether in the vertical direction so as to provide the required depthof girder, and to this end the panels are generally of a rectangularconfiguration. A common type of lattice panel consists of upper andlower longitudinally extending chords which are joined together by alattice of web members These will generally include vertically extendingweb members and angled web members which may for example extend at anangle of about 45′ to the chords. Various panel configurations aredisclosed in GB-A-2251018, for example.

[0003] To join the panels together in end to end relationship to providethe required length of girder, pin and aperture joints are usuallyemployed to ensure speed and simplicity of assembly on site One end ofeach chord will be formed with a female portion and one with a maleportion. These are mated when the panels are to be joined together andthen a transverse pin is inserted through apertures in the portions.

[0004] Bridges of the above type are often used as temporary structures,for example to replace bridges destroyed by floods, earthquakes or actsof war They are supplied as prefabricated components which are assembledon site. In one common method of construction, the bridge girders areassembled on one side of the gap to be bridged, such as a river orravine, and pushed out and over to the other side.

[0005] In a conventional system, the lattice panels are provided ascompletely prefabricated units with the web members welded to thechords. Typically, a standard length prefabricated panel will beavailable, and the designer of a bridge will use the appropriate numberof these, to be joined end to end, for the bridge.

[0006] A significant advantage of using prefabricated panels is thatbridges can be constructed quickly with the minimum of on-sitefabrication however, there are design constraints due to the limitednumber of panel configurations available. It is also necessary torestrict the length of the prefabricated panels used as the basic units,so that there is sufficient flexibility to achieve a desired length byjoining a number together. however, the joins between adjacent panelscan be expensive, particularly if they are of the pin joint varietyrequiring forgings. The more panel units are required to span a givenlength, the more joins are required. a further problem is thatprefabricated panels are bulky to transport.

[0007] An object of the present invention is to provide a modular systemfor constructing lattice panels which provides greater flexibility butwhich does not increase to an undesirable level the time spent on siteto construct a bridge.

[0008] Viewed from one aspect the present invention provides a modularsystem for the construction of a lattice panel for a structure,comprising first and second elongate chord members and a plurality ofweb members for attachment to the chord members so as to hold them aparttransversely, wherein each web member comprises three legs forming atriangle, a first leg being adapted to extend perpendicularly betweenthe chord members, and second and third legs being joined to each otherand to adjacent the ends of the first leg, and wherein there areprovided the following means for interlocking each web member to thechord members and to like web members:

[0009] first interlocking means adjacent one end of the first leg tointerlock with corresponding second interlocking means provided atintervals along the first chord member;

[0010] third interlocking means adjacent the other-end of the first legto interlock with corresponding fourth interlocking means provided atintervals along the second chord member;

[0011] fifth interlocking means adjacent the join between the second andthird legs; and

[0012] sixth Interlocking means intermediate the ends of the first leg;

[0013] the arrangement being such that the fifth interlocking means ofone web member is adapted to interlock with the sixth interlocking meansof an adjacent like web member.

[0014] Thus, in use a lattice panel can be constructed with upper andlower chord members and a lattice of web members providing vertical legsextending between and connected to the chord members, and inclined legswhich are joined to the vertical legs of adjacent web members. Such anarrangement provides a strong lattice panel.

[0015] It is a simple matter to provide lattice panels of desiredlengths, by selecting chord members of appropriate lengths and acorresponding number of web members. To cover a certain distance it maybe possible to use a single panel constructed from the modularcomponents with long chord members, rather than two or more standardlattice panels joined end to end. From a manufacturing point of view, itis preferable to fabricate and store the modular components capable offorming lattice panels of various lengths, rather than complete latticepanels of various lengths Furthermore, the conventional joints betweenlattice panels, typically involving forged components, are expensive.The ability to construct longer panels, simply and from prefabricatedcomponents, reduces the total number of panels required for a particularjob and thus the number of expensive joints The end user will also havefewer inter-panel joints to assemble if longer panels can be used.

[0016] The fabrication of the lattice panels may be carried out at amanufacturing site once an order is received, for shipment to a place ofuse. Alternatively, the modules may be shipped to the end user forassembly into panels on-site This may be preferable from a shippingpoint of view. It may also be possible for the chord members to bemanufactured locally if their design is simple enough, this beingdiscussed below, so that only the web members have to be shipped.

[0017] It is known for the upper chords of lattice panels to be joinedby simple compression joints, for example using abutting flanges andthreaded fasteners. These chords can be made on site relatively easily.The lower chord joints are in tension and conventionally they have beenin the form of pin joints which provide the required tensile strengthwhilst being relatively quick and simple to assemble. The pin joints areexpensive, forged items, and it is less feasible to manufacture thelower chords on site. In accordance with the present invention, however,it is practical to use longer chords and fewer chord joints. Forexample, a panel in accordance with the invention may be between threeand four times the length of a conventional panel. It is thus feasibleto use alternative chord joints for the lower chords, which are cheaperand easier to manufacture even though it may take longer to join twochords. Thus, in one proposed arrangement the lower chord joints areprovided by splice plates and several threaded fasteners An advantage ofsuch a simple joint is that the chords only need to be provided withapertures for the fasteners and it is a more practical proposition forthe chords to be made on site.

[0018] There may be a number of different chords that can be used Forexample, a stronger chord could be provided if required. This could beonly at suitable positions, such as at the centre of a bridge span. Thisavoids the need to take a standard lattice panel and add a reinforcingchord to it. It is also possible to introduce camber by using chords ofdifferent lengths at the top and bottom of a panel. A longer upper chordwill introduce positive camber, resisting the tendency of a bridge tosag in the middle.

[0019] The interlocking means should be such that the modules can beassembled in the required orientation and also provide for the transferof forces. In a preferred embodiment, where two interlocking meansinterlock, there is provided a male spigot on one member and a matchingfemale recess, for example in the form of an aperture, on the othermember. For any interlock there may be one, two, three, four or moresuch spigots and matching recesses. There may also be fasteners such asthreaded bolts secured by nuts to clamp the components together,although the primary purpose of such threaded fasteners will generallynot be to transfer forces. In one preferred form, the first and thirdinterlocking means, at opposite ends of the first leg of the web member,comprise spigots. This means that the chords only need to have aperturesto constitute the corresponding second and fourth interlocking meansThis again helps in simplifying the design of the chords, reducingmanufacturing costs and also making it feasible for them to bemanufactured on site. However, if desired, reinforcing components couldbe attached to the chords to receive the spigots.

[0020] The fifth and sixth interlocking means, which are provided tojoin the web members together, preferably also comprise spigots andrecesses. For ease of manufacture, at least one of the interlockingmeans may be provided on a cast member.

[0021] In general, the philosophy behind the preferred implementation ofthe invention is to keep the chords and their joints as simple aspossible, and to concentrate the more complex and/or expensivestructures on the web members. The design of the modular web member maylend itself to robotic construction, something which has not beenconsidered feasible with the construction of entire lattice panels fromindividual components.

[0022] A web member may be in the form of an isosceles triangle, withthe second and third legs of equal length. In such an arrangement, thesixth interlocking means will be arranged on the mid point of the firstleg and the fifth interlocking means, where the second and third legsmeet, will be aligned with this. In one preferred arrangement, the anglethat each of the second and third legs makes with the first legs isabout 45°, so that the width of a web member is about one half of itsheight, i.e the extent of the first leg which extends perpendicularlybetween the chord members. By using a web member with a longer firstleg, it is possible to construct a deeper lattice panel. Preferably,when this is done the angles between the first leg and the second andthird legs are preferably reduced to retain the same width for the webmember so that it will remain compatible with the same chords, decks andother components used with other web members. In general, the anglebetween the first leg and the second/third legs is preferably in therange of 35° to 45°. In preferred embodiments, this range can providedpanels in the range of about 15 feet (about 4.5 m ) to about 23 feet(about 7 m ) high Conventional Bailey bridge panels are frequentlystacjed on each other to increase height, and apart from anything elsethis doubles the number of chord joints that have to be made

[0023] It will be appreciated that in a practical arrangement thetriangle may not be perfect and that the legs might not consist solelyof members which meet immediately adjacent their ends, for example.Thus, in one preferred arrangement the second and third legs may bejoined together by a junction unit which receives the ends of both legsand is provided with the fifth interlocking means. Similarly, the firstleg may . comprise a member which is joined at each end to a junctionunit. These are respectively attached to the second and third legs, andhave the first and third interlocking means for connection to the chordmembers.

[0024] A typical chord member for use in accordance with the presentinvention be of H section. Such a section will effectively define a pairof channels. One end of a first leg of a web member (in practice, ajunction unit) will be received within one channel section of an upperchord, and the other end of the first leg will be received within onechannel section of a lower chord.

[0025] The web member first legs may each comprise a pair of spaced,parallel elements. These will help to resist outwards deflection of thechords, and in particular the upper chord when a panel is used in abridge. The spaced elements are preferably tube members, as indeed arethe legs of the web members generally. The use of such web members,resistant to deflection, means that there will normally need to be onlyone line of panels along a side of a bridge, with only a single upperchord and a single lower chord. Frequently in traditional Bailey bridgestructures it is necessary to have twin lines of panels. This thereforedoubles the number of chord joints and this is another reason why inpreferred embodiments of the present invention it is feasible to usechord joints which, individually, take longer to assemble. The preferredweb members, being more resistant to deflection, may also make itunnecessary to use additional lateral struts which are frequently usedin conventional Bailey bridge structures.

[0026] The chord members may be provided with any suitable means forinterconnecting them to the chord members of adjacent panels. Thisinclude male and female pin joint portions, for receiving eithervertical or horizontal pins; apertured plates for receiving bolts orother suitable fasteners as disclosed in GB-A-2 251 018 for example; orany other suitable means. However, as noted earlier, a preferred jointfor the upper chords is a compression joint using flanges and fasteners,and a preferred joint for the lower chords is a tension joint usingsplice plates and fasteners. This also has the advantage that such ajoint may more resistant to fatigue, as it does not require the use ofwelding to attach forged pin joint components to the chords.

[0027] When a lattice panel is constructed using the chord members andweb members as described above, at one end there may be the second andthird legs of a web member projecting beyond the ends of the chords; andat the other end there will then be a first leg of another web memberpositioned inwardly of the ends of the chords. Joining two latticepanels together will involve joining the upper and lower chordstogether, and also joining the projecting web member to the web memberof the adjacent panel.

[0028] For use in a bridge or similar structure, preferably the firstleg of a web member is provided with means for attachment to a transomwhich will support a deck. The connection between the leg and thetransom may be by means of a spigot and recess, for example atrapezoidal cross section recess on the first leg and a matching spigoton the transom. In practice, it may only be necessary to attach atransom to alternate web members. The web members which are not to beattached to transoms may not be provided with the necessary attachmentmeans, and they may even have lighter first legs as they will berequired to withstand less stress than the first legs of the other webmembers. Where s transom is attached, the upright first legs form theuprights of a stress transmitting “U”. At the ends of a structure, therecould be stronger web members with sturdier upright legs and if desiredalso sturdier diagonal legs, to account for increased shear forces.These end web members could be provided with means for attachment totransoms. Other web members could be provided for various purposes asrequired. For example, a special web member adapter could be provided sothat a bridge can be launched using a conventional launcher nose usedwith current Bailey type bridges.

[0029] The invention may be viewed from various different aspects,dealing with the system in broad terms, a web member for use in thesystem, novel lattice panels constructed using the system, a bridge orother lattice panel structure such as a tower constructed using thesystem, a method of constructing such a structure, and so forth.

[0030] For example, viewed from one aspect the present inventionprovides a prefabricated web member for use in a system as describedabove, comprising three legs forming a triangle, wherein:

[0031] a first leg comprises an elongate member having at each endrespective first and second mounting plates perpendicular to the axis ofthe first leg, each mounting plate being provided with firstinterlocking means for connection to a chord member and with at leastone aperture to receive a fastener to secure the mounting plate to thechord member;

[0032] the second and third legs extend at an acute angle from adjacentthe ends of the first leg to a junction where there is provided a thirdmounting plate whose plane is parallel to the longitudinal direction ofthe first leg, the third mounting plate being provided with secondinterlocking means for connection to another web member and with atleast one aperture to receive a fastener to secure the mounting plate tothe other member; and

[0033] the first member is provided with a fourth mounting plateintermediate its ends whose plane is parallel to the longitudinaldirection of the first leg, the plate being provided with thirdinterlocking means for connection to the second interlocking means ofanother web member, and also being provided with at least one apertureto receive a fastener to secure the plate to the other web member.

[0034] Some embodiments of the invention will now be described by way ofexample and with reference to the accompanying drawings in which .

[0035]FIG. 1 is a perspective view of a web member for use in a systemin accordance with the invention;

[0036]FIG. 2 is a side view of part of the web member in the region Amarked on FIG. 1;

[0037]FIG. 3 is an underneath plan view of part of an upper chord usedin the system;

[0038]FIG. 4 is a section through the part of the upper chord;

[0039]FIG. 5 is a section through part of a lower chord;

[0040]FIG. 6 is a side view of a lattice panel using the web members andchords;

[0041]FIG. 7 is a perspective view of part of a bridge constructed usinga number of the lattice panels;

[0042]FIG. 8 is a perspective view of a modified web member;

[0043]FIG. 9 is a plan view of an upper chord for use with the webmember of FIG. 8;

[0044]FIG. 10 is an end view of the chord of FIG. 9;

[0045]FIG. 11 is a plan view of a lower chord for use with the webmember of FIG. 8;

[0046]FIG. 12 is an side view of the chord of FIG. 11 ;

[0047]FIG. 13 is perspective view of an end post for use in a systemwith the components of FIGS. 8 to 12; and

[0048]FIG. 14 is a perspective view of part of a bridge using thecomponents of FIGS. 8 to 13.

[0049] In FIG. 1 a steel web member 1 is of generally triangular shape,having an elongate upright leg 2 and two legs 3 and 4 of equal length,inclined at about 45° to the upright leg. The upright leg 2 comprises apair of spaced, parallel, square section tubes 5 and 6 At the upper endof the leg 2 is a junction unit 7 which is welded between the tubes 5and 6. This includes a pair of spaced vertical plates 8 between which iswelded the end of leg 4, and a horizontal plate 9. The plane ofhorizontal plate 9 is therefore perpendicular to the elongate axis ofupright leg 2. The plate 9 has three apertures 10 for receivingfasteners, and a large central aperture 11 for receiving a locating lugto interlock the web member to an upper chord. At the lower end of theupright leg 2 is a corresponding junction unit 12, which receives theend of leg 3 and is adapted to be connected to a lower chord. This has aplate and apertures corresponding to those in the upper unit 7.

[0050] The other ends of legs 3 and 4 are received by a third junctionunit 13 this comprises a pair of spaced vertical plates 14, betweenwhich the ends of the legs are welded, and a vertical plate 15. Thevertical plate 15 comprises a pair of apertures 16 for receivingfasteners, and three large, vertically spaced apertures 17 for receivinglocating lugs on a like web member. The junction unit 13 is positionedvertically mid way relative to the upright leg 2.

[0051] Mid way up the upright leg 2 is provided a mounting plate 18,welded to the tubes 5 and 6. With reference to FIG. 2, this is providedwith three vertically spaced lugs 19 which are adapted to mate with theapertures 17 on a plate 15 of a like member. Apertures 20 are provided,to be aligned with apertures 16 on the plate 15 of a like member, sothat the two plates 18 and 15 can be secured together, e.g. by means ofthreaded fasteners and nuts, and thus two web members joined together.

[0052]FIG. 3 is an underneath view of part of a steel upper chord 20 ofH section, and FIG. 4 is a section through part of the chord. Spacedalong the upper chord 20 at equal intervals are location means in theform of plates 21. Each plate 21 has three apertures 22 for receivingthreaded fasteners, and a downwardly projecting lug 23. The lug 23 isconfigured to locate inside the aperture 11 in plate 9, on the end ofleg 2 of a web member. The apertures 22 will then be aligned with theapertures 10 in the plate so that the web member can be attached to theupper chord by means of nuts and bolts. The junction unit 7 on the webmember fits in the space between the flanges 24 and 25 of the lower partof the “H” section of the upper chord 20. At each end of the upper chordthere is provided a transverse plate 26 which is apertured at 27 (FIG.7) so that two chords of adjacent panels can be joined together byabutting the plates 26 and securing them by means of bolts passingthrough the apertures.

[0053]FIG. 5 shows a lower chord 28 in section, this also being a steelH section member. This has plates 29 spaced at equal intervals along itslength, defining lugs 30 and apertures 31. These are adapted tocooperate with corresponding apertures in the lower junction unit 12 ofthe leg 2 of a web member, in the same way that the upper unit 7 issecured to the upper chord 20. In this manner a web member 1 can besecured between the upper and lower chords, with the upright leg 2extending perpendicularly between them.

[0054] It will be appreciated that in the above arrangement, the webmembers are secured to the transverse portions of the “H” section upperand lower chords. This means that the width of the chords can be varied,for example so as to increase or decrease their strength, withoutaffecting the connections with the web members. With a conventionalstructure, the web forming members are frequently connected to thevertical flanges forming the legs of the H section chords. Thus, in suchconventional arrangements, varying the widths of the chords would varythe spacing between the flsnges to which the web forming members need tobe attached. It will also be appreciated that with the new arrangementdescribed above, it is possible to use a single chord with web membersof different widths, provided they fit in the space between the verticalflanges.

[0055] It is possible to mix the widths of the chords used in aparticular bridge, for example to increase strength where there is highloading. It should be noted that the compression type of joint used onthe upper chords facilitates this. Apertures can be aligned in thetransverse end plates 26 of the upper chords, even if the overall widthsare different.

[0056] The lower chord has pin joint portions at its end for joining toadjacent lower chords, in this arrangement comprising a pair of maleportions 32 at one end (FIGS. 6 and 7) and a pair of female portions 33at the other end. The portions can be joined together by vertical pins34 (FIG. 7).

[0057]FIG. 6 is a side view of a lattice panel 3S comprising upper chord20, lower chord 28 and four web members 1 Each of these is joined to thechords as described above, so that the upright legs 2 extendsubstantially perpendicularly between the chords. The web members 1 arealso joined to each other. The lugs 19 on the upright leg 2 of one webmember are engaged in the apertures 17 of the junction unit 13 of anadjacent panel, and the web members are joined together by nuts andbolts through the mating apertures 16 and 20.

[0058] To construct a bridge member, a number of panels 35 are joinedend to end. The plates 26 of adjacent upper chords, and the pint jointportions 32 and 33 of adjacent lower chords are joined as describedabove. In addition, the junction unit 13 of the protruding web member ofone panel mates with the lugs 19 of the adjacent panel, and the two webmembers are joined together as described above.

[0059] As shown in FIG. 7, a number of panels 35 are joined together endto end to form a left hand side member of a bridge, and a number arealso joined together end to end to form a right hand side member.Transverse supports 36 are attached to the panels, by means of junctionblocks 37 which are provided on each upright web member leg 2, weldedbetween the tubes 5 and 6. The transverse supports are secured bythreaded fasteners. Decking 38 is laid over the transverse supports 36.The junction blocks 37 have trapezoidal recesses which receive spigotson the transverse supports As a threaded fastener is tightened, it urgesa spigot into the tapered trapezidal recess thus tightening theengagement between the spigot and recess. This reduces play in theconnection between the trabnsverse suppports 36 and the web members andreduces misalignment. Misalignment can reduce the stability of the upperchord in particular.

[0060] As described above, four web members 1 are used for each panel35, However, longer or shorter chords can be used, and more or fewer webmembers, so as to produce panels of different lengths.

[0061] FIGS. 8 to 14 illustrate a modified system. Many components arethe same and their description is not repeated FIG. 8 shows a modifiedweb member 40 with legs 41, 42 and 43. At either end of leg 41 areinterlocking means 44 and 45 for uae with upper and lower chordsrespectively. Each interlocking means is provided with four spigots 46and three apertures 47 for receiving fasteners. FIG. 9 shows an upperchord 48 for use with the modified web member 40. Along its length areinterlocking means each comprising four apertures to receive spigots 46and three apertures to match apertures 47 and receive threadedfasteners. As shown in FIG. 10, the end of the chord member is providedwith a plate 51 having apertures 52, so that it can be attached to alike chord to form a compression joint.

[0062]FIG. 11 shows a bottom chord 53 which like chord 48 hasinterlocking means along its length comprising apertures 54 to receivespigots of interlocking means 45 of the web member 40 and apertures 55to receive threaded fasteners. At its end it is provided with twelveapertures 56 and twelve side apertures 57 (FIG. 12) so that it can beattached to a like chord by a splice plate and fasteners passing throughthe apertures.

[0063]FIG. 13 shows an end post 58 for use in a system with web member40 and chords 48 and 53 It has the same interlocking means 59, 60 at itsends It also has additional connectors 61 which can be used to attach alaunch “nose” of a conventional type for when a bridge is being pushedout over a river or the like.

[0064]FIG. 14 shows part of a bridge 62 using the components of FIGS. 8to 13. It shows web members 40, upper chords 48 joined at 63, lowerchords 53 joined at 64, end posts 58, transoms 65 extending between theweb members and between the end posts, and part of a deck 66 laid on thetransoms.

1. A modular system for the construction of a lattice panel for astructure, comprising first and second elongate chord members and aplurality of web members for attachment to the chord members so as tohold them apart transversely, wherein each web member comprises threelegs forming a triangle, a first leg being adapted to extendperpendicularly between the chord members, and second and third legsbeing joined to each other and to adjacent the ends of the first leg,and wherein there are provided the following means for interlocking eachweb member to the chord members and to like web members: firstinterlocking means adjacent one end of the first leg to interlock withcorresponding second interlocking means provided at intervals along thefirst chord member; third interlocking means adjacent the other end ofthe first leg to interlock with corresponding fourth interlocking meansprovided at intervals along the second chord member; fifth interlockingmeans adjacent the join between the second and third legs; and sixthinterlocking means intermediate the ends of the first leg; thearrangement being such that the fifth interlocking means of one webmember is adapted to interlock with the sixth interlocking means of anadjacent like web member
 2. A system as claimed in claim 1, wherein theweb member is in the form of an isosceles triangle, with the second andthird legs oaf equal length
 3. A system as claimed in claim 2, whereineach of the second and third legs is inclined to the first leg at anangle of between about 35° to 45°.
 4. A system as claimed in claim 1, 2or 3 wherein the second and third legs are joined together by a junctionunit which receives the ends of both legs and is provided with the fifthinterlocking means.
 5. A system as claimed in any preceding claim,wherein the first leg is provided at each end with a junction unit, onebeing attached to the second leg and having the first interlocking meansfor connection to a chord member, and one being attached to the thirdleg and having the third interlocking means for connection to a chordmember.
 6. A system as claimed in any preceding claim wherein one of thefirst and second interlocking means comprises a spigot and the othercomprises a recess adapted to receive the spigot.
 7. A system as claimedin claim 6, wherein said one of the first and second interlocking meanscomprises a plurality of spigots.
 8. A system as claimed in claim 6 or7, wherein the said one of the first and second interlocking means isthe first interlocking means.
 9. A system as claimed in claim 6, 7 or 8wherein the first and second interlocking means further comprise alignedapertures for receiving fasteners to secure the web member to the chordmember.
 10. A system as claimed in any preceding claim wherein one ofthe third and fourth interlocking means comprises a spigot and the othercomprises a recess adapted to receive the spigot.
 11. A system asclaimed in claim 10, wherein said one of the third and fourthinterlocking means comprises a plurality of spigots.
 12. A system asclaimed in claim 10 or 11, wherein the said one of the third and fourthinterlocking means is the third interlocking means.
 13. A system asclaimed in claim 10, 11 or 12, wherein the third and fourth interlockingmeans further comprise aligned apertures for receiving fasteners tosecure the web member to the chord member.
 14. A system as claimed inany preceding claim wherein one of the fifth and sixth interlockingmeans comprises a spigot and the other comprises a recess adapted toreceive the spigot.
 15. A system as claimed in claim 14, wherein thefifth and sixth interlocking means further comprise aligned aperturesfor receiving fasteners to secure one web member to an adjacent webmember.
 16. A system as claimed in claim 14 or 15, wherein one of thefifth and sixth interlocking means comprises a plurality of verticallyspaced spigots and the other comprises a corresponding plurality ofapertures adapted to receive the spigots.
 17. A system as claimed in anypreceding claim, wherein the first leg comprises a pair of parallel,spaced elements.
 18. A system as claimed in any preceding claim whereinthe first leg of at least some of the web members comprises means forattachment to a transom.
 19. A system as claimed in any preceding claimwherein one of the chord members is provided with apertured flanges atits ends to form compression joints with chord members of like panels bythe use of fasteners passing through the apertures.
 20. A system asclaimed in any preceding claim, wherein one of the chord members isprovided with apertures at its ends to form tension joints with chordmembers of like panels by the use of splice plates and fasteners passingthrough the splice plates and the apertures.
 21. A system as claimed inclaim 17 wherein the parallel, spaced elements are tubular.
 22. A systemas claimed in claim 1, wherein the first leg of the web member comprisesan elongate member having at each end respective first and secondmounting plates perpendicular to the axis of the first leg, one mountingplate being provided with the first interlocking means for connection toa chord member and with at least one aperture to receive a fastener tosecure the mounting plate to the chord member, and the other mountingplate being provided with the third interlocking means for connection toa chord member and with at least one aperture to receive a fastener tosecure the mounting plate to the chord member; the second and third legsof the web member extend at an acute angle from adjacent the ends of thefirst leg to a junction where there is provided a third mounting platewhose plane is parallel to the longitudinal direction of the first leg,the third mounting plate being provided with the fifth interlockingmeans for connection to another web member and with at least oneaperture to receive a fastener to secure the mounting plate to the othermember; and the first member of the web member is provided with a fourthmounting plate intermediate its ends whose plane is parallel to thelongitudinal direction of the first leg, the plate being provided withthe sixth interlocking means for connection to the second interlockingmeans of another web member, and also being provided with at least oneaperture to receive a fastener to secure the plate to the other webmember.
 23. A lattice panel constructed from a system as claimed in anypreceding claim, comprising the upper and lower chord members and aplurality of the web members connected to the chord members and to eachother.
 24. A structural member comprising a plurality of panels asclaimed in claim 23 joined end to end, with the upper chords of adjacentpanels connected together, the lower chords of adjacent panels connectedtogether, and a web member of one panel being connected to a web memberof an adjacent panel by means of the fifth interlocking means of the webmember of one panel being interlocked with the sixth interlocking meansof the web member of the other panel.
 25. A structure comprising astructural member as claimed in claim
 24. 26. A structure as claimed inclaim 25, in the form of a bridge.
 27. A prefabricated web member foruse as the web member in a system as claimed in any of claims 1 to 22,comprising three legs forming a triangle, wherein: a first leg comprisesan elongate member having at each end respective first and secondmounting plates perpendicular to the axis of the first leg, one mountingplate being provided with the first interlocking means for connection toa chord member and with at least one aperture to receive a fattener tosecure the mounting plate to the chord member, and the other mountingplate being provided with the third interlocking means for connection toa chord member and with at least one aperture to receive a fastener tosecure the mounting plate to the chord member; the second and third legsextend at an acute angle from adjacent the ends of the first leg to ajunction where there is provided a third mounting plate whose plane isparallel to the longitudinal direction of the first leg, the thirdmounting plate being provided with the fifth interlocking means forconnection to another web member and with at least one aperture toreceive a fastener to secure the mounting plate to the other member; andthe first member is provided with a fourth mounting plate intermediateits ends whose plane is parallel to the longitudinal direction of thefirst leg, the plate being provided with the sixth interlocking meansfor connection to the second interlocking means of another web member,and also being provided with at least one aperture to receive a fastenerto secure the plate to the other web member.
 28. A web member as claimedin claim 27, wherein the second and third legs are of substantiallyequal lengths.
 29. A web member as claimed in claim 28, wherein each ofthe second and third legs extends at from about 35° to 45° to the firstleg.
 30. A web member as claimed in claim 27, 28 or 29, wherein one ofthe fifth and sixth interlocking means comprises a spigot and the othercomprises a recess adapted to receive the spigot.
 31. A woo member asclaimed in claim 30, wherein one of the fifth and sixth interlockingmeans comprises a plurality of vertically spaced spigots and the othercomprises a corresponding plurality of apertures adapted to receive thespigot.
 32. A web member as claimed in any of claims 27 to 31 whereineach of the first and third interlocking means comprises a spigot.
 33. Aweb member as claimed in any of claims 27 to 32, wherein the first legcomprises a pair of parallel, spaced elements;
 34. A web member asclaimed in claim 33 wherein the parallel, spaced elements are tubular